1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses with the entire lens system being reduced in size and weight so as to be particularly to video cameras.
2. Description of the Prior Art
There has been the trend of advancing minimization of the length of zoom lenses in longitudinal and lateral directions and, zoom lenses of relatively small size for 35mm cameras are proposed in, for example, Japanese Laid-Open Patent Application No. Sho 59-16248, U.S. Pat. No. 4,159,865, and U.S. Pat. Application Ser. No. 733,856, now U.S. Pat. No. 4,746,160.
These publication disclose a so-called 2-unit form of zoom lens of which the first lens unit, counting from the front, is of negative power and the second lens unit is of positive power. When zooming, the first and second lens units are axially moved, while varying their air separation, respectively non-linearly and linearly forward.
In this 2-unit zoom lens, the first lens unit has two functions i.e. focusing and compensating for the shift of the image plane that accompanies the variation of the magnification, and the second lens unit has two functions i.e. varying the magnification and forming an image. Since the 2-unit form is relatively favorable to increase the image angle of the zoom lens, it has been widely used in wide angle zoom lenses.
It has been the general practice in the art of 2-unit zoom lenses that a diaphragm movable with zooming and capable of determining the F-number of the lens is located within the second lens unit.
Therefore, in a case where the 2-unit zoom lens is mounted on a video camera, a mechanism for driving the diaphragm is apt to become complicated. The reason for this is that, in such a video camera, an actuator such as one meter coupled to the diaphragm is generally controlled on the basis of a video signal produced by an image pickup tube.